Magnetic Biochar Obtained by Chemical Coprecipitation and Pyrolysis of Corn Cob Residues: Characterization and Methylene Blue Adsorption

Biochar is a carbonaceous and porous material with limited adsorption capacity, which increases by modifying its surface. Many of the biochars modified with magnetic nanoparticles reported previously were obtained in two steps: first, the biomass was pyrolyzed, and then the modification was performed. In this research, a biochar with Fe3O4 particles was obtained during the pyrolysis process. Corn cob residues were used to obtain the biochar (i.e., BCM) and the magnetic one (i.e., BCMFe). The BCMFe biochar was synthesized by a chemical coprecipitation technique prior to the pyrolysis process. The biochars obtained were characterized to determine their physicochemical, surface, and structural properties. The characterization revealed a porous surface with a 1013.52 m2/g area for BCM and 903.67 m2/g for BCMFe. The pores were uniformly distributed, as observed in SEM images. BCMFe showed Fe3O4 particles on the surface with a spherical shape and a uniform distribution. According to FTIR analysis, the functional groups formed on the surface were aliphatic and carbonyl functional groups. Ash content in the biochar was 4.0% in BCM and 8.0% in BCMFe; the difference corresponded to the presence of inorganic elements. The TGA showed that BCM lost 93.8 wt% while BCMFe was more thermally stable due to the inorganic species on the biochar surface, with a weight loss of 78.6%. Both biochars were tested as adsorbent materials for methylene blue. BCM and BCMFe obtained a maximum adsorption capacity (qm) of 23.17 mg/g and 39.66 mg/g, respectively. The obtained biochars are promising materials for the efficient removal of organic pollutants.


Introduction
Due to rapid population growth, accelerated urbanization, and industrialization, the volume and diversity of agro-industrial wastes have increased. These wastes are considered a significant source of pollution due to poor management and improper disposal [1,2] (Awogbemi and Kallon, 2022; Karić et al., 2022). A strategy to reduce the disposed residues is to prepare new materials for different applications. Biochar is a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment [3]. It is a carbon-rich organic material that is prepared by heating biomass. Depending on the feedstock used to obtain it, the properties of the biochar such as its high specific surface area, microporosity, and ion exchange capacity can be improved [4,5]. Furthermore, corn

Chemical Coprecipitation of Magnetite in Corn Cob Waste
The coprecipitation method was based on the work of Rodriguez et al. (2019) [24]. A solution was prepared in a beaker with 50 mL of deionized water, FeCl 3 ·6H 2 O and FeCl 2 ·4H 2 O at a 2:1 ratio according to Equation (1): The solution was heated at 50 • C for 15 min. Corn cobs cleaned of waste were placed in contact with 5 mL of Fe 3+ :Fe 2+ ion solution. Then, 2 mL of concentrated NaOH was added by dripping and subsequently dried at 100 • C for 24 h.

Obtaining Biochar from Corn Cob by Pyrolysis
The research of Lee et al. (2017) and Xie et al. (2021) was used to develop corn cob pyrolysis [25,26]. Dried corn cob residues were placed in a CARBOLITE™ CTF horizontal tube furnace (Fischer Scientific, Monterrey, MX) and heated to 500 • C for 1 h. The heating rate and N 2 flow rate were varied. The obtained sample was allowed to cool slowly at room temperature. Biochar was labeled as BCM, and the modified biochar as BCM Fe .

Characterization of Biochar
The biochar yield was determined according to the procedure reported by Qin et al. (2020), as well as the ash content using ASTM D2866-94 (ASTM, n.d.) [27,28]. In addition, SEM and EDS analysis were performed on a HITACHI model SU8230 cold cathode SEM machine (Ciudad de México, MX) with backscattered electron and secondary electron detectors; FTIR analysis with a PerkinElmer FTIR Spectrometer Frontier (Monterrey, Mexico) in a range of 4000 to 600 cm −1 using 32 scans and 4 cm −1 resolution; XRD on a Bruker D8 ADVANCE model (Ciudad de México, MX) (CuKa: 1.54 Å, 40 mA and 40 kV); XRF analysis on a Malvern Panalytical Epsilon 1 model (Monterrey, MX); and TGA on a TA Instruments TGA 550 (Ciudad de México, MX), in a temperature range of 30 • C to 700 • C at a heating rate of 10 • C/min, using air atmosphere; and N 2 physisorption at 77 K was performed in a BEL BELSORP-miniX surface analyzer (Ciudad de México, MX).

Methylene Blue Adsorption Efficiency and Kinetic Models
MB adsorption efficiency was determined for both biochars. In 15 mL conical tubes, 0.02 g of biochar and 10 mL of an MB solution at different concentrations (i.e., 1, 2, 4, 8, 10, and 25 ppm) were placed in contact. The pH was adjusted to 7 ± 0.2 in an Orion StarA2110 potentiometer (Monterrey, MX). The initial concentration (C o ) absorbance of each of the solutions was measured in a Mettler Toledo UV-Vis instrument model UV5 (Ciudad de México, MX) (λ = 664 nm). Then, the tubes were placed in a LabTech LSI-3016A incubator (El Crisol, Monterrey, México) at 25 • C and 150 rpm for 24 h. Once equilibrium was reached, the remaining concentration (C e ) values were measured by UV-Vis. The maximum adsorption capacity was calculated according to Equation (2) and fitted to the Langmuir isotherm model according to Equation (3): where: q e = adsorption capacity at equilibrium, mg·g −1 C o = initial concentration of MB solution, mg·L −1 C e = final concentration of MB solution, mg·L −1 V = solution volume, L m = adsorbent mass, g For kinetic experiments, 60 mL of a 10 ppm MB solution and 60 mg of the material were placed in a beaker. Sampling was carried out during 0, 1, 3, 3, 5, 10, 20, 40, and 60 min. The concentration was measured by a UV/Vis spectrometer. The data obtained were adjusted to the pseudo-second-order model using the following equation (Equation (4)): where: t = time, min q t = adsorption capacity at time, mg·g −1 q e = adsorption capacity at equilibrium, mg·g −1 k 2 = PSO constant (g·mg −1 ·g −1 )

Biochar Yield and Ash Percentage
It is important to dry the corn cob samples prior to pyrolysis in order to calculate the amount of biochar obtained. Besides, subjecting wet feedstock samples to pyrolysis would consume energy, and carbonization would last longer [8]. In addition, a large amount of moisture in the biomass leads to the production of liquid by-products [29]. The pyrolysis experimental conditions of the BCM samples and the yield are shown in Table 1. The N 2 flow rate was tested, and for a 50 mL/min rate, the yield was 5.60%, and for a 35 L/min rate, the yield was 28.48%. Moreover, different heating ramps were tested. The results showed that when a lower heating ramp was used (5 • C/min), the biochar yield was 26.91%. However, a higher heating ramp (10 • C/min) produced a biochar yield of 28.48% and reduced the time needed to reach the pyrolysis temperature. In addition, raw and cooked corn cob (180 • C, 1 h) conditions were tested, and no significant difference in biochar yield was observed. Therefore, to obtain a higher biochar yield, an N 2 flow rate of 35 mL/min and a heating ramp of 10 • C/min were used, in which an average yield of 28.48 ± 0.098% was obtained. In Table 2, the ash contents of different samples are shown. It can be observed that corn cob residues contain 0.4%, while BCM shows a significant increase with 4.0%, which is lower compared to what was reported by Zhao et al. (2018); these authors determined 10.79% of ash content when heating at 300 • C and 23.27% at 500 • C [30]. The results shown in Table 2 are also different from those reported by Mohan et al. (2014) and Xie et al. (2021), who used corn as biomass and the same carbonization temperature [26,31]. For BCM Fe , an 8.0% ash content was observed, a higher amount compared to BCM, which was attributed to the presence of inorganic particles, such as iron oxides, on the surface of BCM Fe , due to the melting point of iron being >1500 • C.
The biochar yield varied according to the type of biomass and the temperature used in the pyrolysis process, according to Amalina et al. (2022) [32]. Table 3 shows a comparison with biochars reported in the literature. For BCM, the results are similar to those reported by Mohan et al. (2014) [31]. However, as reported by Suo et al. (2021), an increase in the percentage can be attributed to a lower carbonization temperature [33].

Inorganic Composition
In order to determine the ash composition, XRF analysis was performed. The results are shown in Table 4, where it can be observed the presence of elements such as K, P, and Si, which are in a higher proportion in the BCM biochar when compared with corn cobs. The amount of K increases considerably by the pyrolysis process, being the main element in BCM with a concentration greater than 11%, similar to that reported by Gómez-Vásquez et al. (2022) [36]. The high amount of elements was attributed to the fertilizer used for plant growth (Land and Water Division, FAO, 2002) since the main elemental composition is K, P, and Si with concentrations greater than 0.6% [37]. Other elements such as Cl, Fe, and S were also detected, albeit in lower concentrations. For BCM Fe , an increase in Fe amount (8.761%) was attributed to the formation of iron oxides by the coprecipitation method.

Morphological Analysis
SEM analysis was performed to characterize the surface morphologies of the samples. Figure 1 shows the BCM surface, where a highly porous structure, a large surface area

Morphological Analysis
SEM analysis was performed to characterize the ples. Figure 1 shows the BCM surface, where a highly area (SBET: 1013.52 m 2 /g), and evenly distributed pores w was determined as mesopores and macropores, as micr to detect, according to the report by Quillope   The surface structure (S BET = 903.67 m 2 /g) of magnetic biochar (BCM Fe ) is shown in Figure 2. Particles with an intense brightness were observed, indicating the presence of an element with a higher atomic mass such as Fe [39]. Fe 3 O 4 particles generally adopted a spherical shape and formed agglomerates, which were distributed over the biochar surface, according to the reports by Ouyang et al. (2017) and J. Yan et al. (2020) [40,41]. It can be determined from the SEM analysis of Figure 2 using Image-Pro Plus software that the average agglomerate diameter is 5.83 ± 2.23 µm. This is in accordance with the value reported by Navarathna et al. (2019), in which the agglomerated particle size was up to 7 µm [42]. The BCMFe surface elemental composition was obt of the sample (see Figure 3) were analyzed to obtain th ( Table 5). Biochar BCM consists mainly of C (carbon) an also detected, although in smaller proportions. The res tained by XRF analysis for Si, K, and Fe. No presence The BCM Fe surface elemental composition was obtained by EDS analysis. Four points of the sample (see Figure 3) were analyzed to obtain the mass percentage of each element (Table 5). Biochar BCM consists mainly of C (carbon) and O (oxygen). Other elements were also detected, although in smaller proportions. The results were consistent with those obtained by XRF analysis for Si, K, and Fe. No presence of Cl was observed in the sample, so Fe ions were transformed into iron oxide. In addition, the Fe percentage increased significantly due to the presence of Fe particles in the biochar. (Table 5). Biochar BCM consists mainly of C (carbon) and O (oxygen). Ot also detected, although in smaller proportions. The results were consist tained by XRF analysis for Si, K, and Fe. No presence of Cl was observ so Fe ions were transformed into iron oxide. In addition, the Fe percent nificantly due to the presence of Fe particles in the biochar.   Table 5.

Structure of Modified Biochar
The crystalline structures of the modified biochar were characterized by X-ray diffraction. The XRD pattern of the BCM Fe is presented in Figure 4, which confirms that the  [43,44].
In Figure 4, an intense signal at 29.29 • due to silicon oxide (SiO 2 ) with crystallographic chart 96-900-0809 can be observed. Since SiO 2 is a compound that is prominent in the inorganic components of corn cob [36], or it may belong to a potassium mineral (K) with crystallographic chart 96-901-1978, these elements can be found in biochar due to the inorganic elements present in the biomass that was not carbonized. The signals for amorphous graphite were also observed at signals between 20 and 30 • and 40 and 50 • .
In order to identify if the material had the characteristic magnetic properties of magnetite, a simple test was carried out with the addition of a magnet to a suspension with 0.1 g of the magnetic biochar, and as can be seen in Figure 5, the material stuck to the magnet due to the magnetic properties.  [43,44].
In Figure 4, an intense signal at 29.29° due to silicon oxide (SiO2) with crystallographic chart 96-900-0809 can be observed. Since SiO2 is a compound that is prominent in the inorganic components of corn cob [36], or it may belong to a potassium mineral (K) with crystallographic chart 96-901-1978, these elements can be found in biochar due to the inorganic elements present in the biomass that was not carbonized. The signals for amorphous graphite were also observed at signals between 20 and 30° and 40 and 50°. In order to identify if the material had the characteristic magnetic properties of magnetite, a simple test was carried out with the addition of a magnet to a suspension with

Surface Functional Groups
An FTIR analysis was performed to observe surface functional groups on the obtained biochars. The results are presented in Figure 6, where the low intensity of the signals, which are attributed to the temperature at which the pyrolysis process was carried out, can be observed. According to Manoharan et al. (2022), the number of functional groups decreases at higher temperatures due to the release of CO 2 [45]. nals, which are attributed to the temperature at which the pyrolysis process w out, can be observed. According to Manoharan et al. (2022), the number of groups decreases at higher temperatures due to the release of CO2 [45].
Both biochars showed similar bands, one observed at 1712 cm −1 , correspon C=O stretching vibration of the carbonyl group [46], which is formed during th zation process, and the double signals between 2800 and 3000 cm −1 , correspond H aliphatic compounds stretching vibrations [35,47]. The band near 500 cm −1 co scribed as a stretching vibration of metallic oxides due to the presence of inor pounds in BCM and BCMFe [48].  Both biochars showed similar bands, one observed at 1712 cm −1 , corresponding to the C=O stretching vibration of the carbonyl group [46], which is formed during the carbonization process, and the double signals between 2800 and 3000 cm −1 , corresponding to -C-H aliphatic compounds stretching vibrations [35,47]. The band near 500 cm −1 could be described as a stretching vibration of metallic oxides due to the presence of inorganic compounds in BCM and BCM Fe [48].

Thermal Behavior
Thermal analysis was carried out on both biochars (BCM and BCM Fe ). The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of Figure 7 show two main weight loss processes for both biochars. In the first curve, BCM showed a 4% loss at 89 • C, while BCM Fe showed a 10% weight loss, due to water molecules evaporating from the biochar surface. At higher temperatures, a greater reduction in weight loss was observed. In BCM, a weight loss of 93.8% was observed at 500 • C, while a weight loss of 78.6% was observed in BCM Fe at 650 • C. The loss was attributed to the decomposition of lignocellulosic biomass and extracts such as proteins, fats, and sugars, among others [14,49], results observed in FTIR analysis, in addition to the loss of functional groups such as carboxyl and hydroxyl groups [50]. Compared to BCM biochar, BCM Fe carbonized at a higher temperature and showed higher thermal stability, which corresponded to a greater amount of inorganic components [51].
as carboxyl and hydroxyl groups [50]. Compared to BCM biochar, BCMFe carbonized at a higher temperature and showed higher thermal stability, which corresponded to a greater amount of inorganic components [51].
The difference between the amount of weight loss for BCMFe and BCM was 15.2% and corresponds to the F3O4 present on the biochar surface according to XRD results. The DTG curve of the impregnated biochar showed two signals between 400 and 650 °C, attributed to the Fe3O4 oxidation to α-Fe2O3 and γ-Fe2O3.

Methylene Blue Adsorption Capacity
MB adsorption capacity was studied at pH 7 since the PZC of BCM was measured as 7.06. The adsorption capacity was adjusted to the Langmuir model, as can be seen in Figure 8. Non-linear regression was used to adjust the experimental data using Origin ® software. BCM biochar showed a maximum adsorption capacity (qm) of 23.17 mg/g and BCMFe biochar of 39.66 mg/g, as shown in Table 6.
The qm of BCMFe biochar is similar to that reported by Zeng et al. (2021), with a qm of 39.35 mg/g at a temperature of 35 °C, using a sewage sludge-based magnetic biochar (Fe3O4) by pyrolysis for the removal of methylene blue in water [22]. Mubarak et al. (2015) prepared a magnetic biochar derived from an empty fruit bunch treated with ferric chloride (FeCl3) to remove methylene blue, obtaining a qm of 31.25 mg/g [52], while Ruthiraan

Methylene Blue Adsorption Capacity
MB adsorption capacity was studied at pH 7 since the PZC of BCM was measured as 7.06. The adsorption capacity was adjusted to the Langmuir model, as can be seen in Figure 8. Non-linear regression was used to adjust the experimental data using Origin ® software. BCM biochar showed a maximum adsorption capacity (q m ) of 23.17 mg/g and BCM Fe biochar of 39.66 mg/g, as shown in Table 6.
The q m of BCM Fe biochar is similar to that reported by Zeng et al. (2021), with a q m of 39.35 mg/g at a temperature of 35 • C, using a sewage sludge-based magnetic biochar (Fe 3 O 4 ) by pyrolysis for the removal of methylene blue in water [22]. Mubarak et al. (2015) prepared a magnetic biochar derived from an empty fruit bunch treated with ferric chloride (FeCl 3 ) to remove methylene blue, obtaining a q m of 31.25 mg/g [52], while Ruthiraan et al.
(2017) obtained a qm of 46.30 mg/g in MB removal with a magnetic iron oxide biochar (impregnated with Fe 2 O 3 solution) obtained from mangosteen peel [53].
In addition to q m , Table 6 shows the k L , R L , and R 2 parameters. The Langmuir constant (k L ) indicates the degree of interaction between the adsorbate and the surface. When the adsorption energy constant (k L ) was less than 1, it indicates that there was a strong interaction between the adsorbate and the adsorbent. The Langmuir separation factor (R L ) showed values lower than 1 in both biochars, which indicated an appropriate adsorption process. If R L > 1 is not suitable, while if 0 < R L < 1 is appropriate [54].   In addition to qm, Table 6 shows the kL, RL, and R 2 parameters. The Langmuir constant (kL) indicates the degree of interaction between the adsorbate and the surface. When the adsorption energy constant (kL) was less than 1, it indicates that there was a strong interaction between the adsorbate and the adsorbent. The Langmuir separation factor (RL) showed values lower than 1 in both biochars, which indicated an appropriate adsorption process. If RL > 1 is not suitable, while if 0 < RL < 1 is appropriate [54].
Further, the linear correlation coefficient (R 2 ) was high, so the adsorption isotherms fit well for the Langmuir model. This model indicates that the adsorbent surface was completely flat and homogeneous and that each surface site can only hold one molecule of the adsorbate, resulting in monolayer-type adsorption [55,56]. The metal-π interaction is one of the mechanisms reported in the literature for organic compounds in modified biochars with metallic oxides [57][58][59][60]. By this, an increase in the MB adsorption capacity for BCMFe was observed, due to the presence of Fe3O4 particles on the surface. Several reports have corroborated that MB adsorption by activated carbons from lignocellulosic materials demonstrates a strong correlation with the Langmuir isotherm equation [61].
Furthermore, experimental data were adjusted to the PFO and PSO order kinetic models; however, both biochars were better fitted to the PSO (R 2 > 0.998) (Figure 9). The PSO model describes the adsorption process in a surface monolayer. The results agreed with those observed in adsorption isotherms.  Further, the linear correlation coefficient (R 2 ) was high, so the adsorption isotherms fit well for the Langmuir model. This model indicates that the adsorbent surface was completely flat and homogeneous and that each surface site can only hold one molecule of the adsorbate, resulting in monolayer-type adsorption [55,56]. The metal-π interaction is one of the mechanisms reported in the literature for organic compounds in modified biochars with metallic oxides [57][58][59][60]. By this, an increase in the MB adsorption capacity for BCM Fe was observed, due to the presence of Fe 3 O 4 particles on the surface. Several reports have corroborated that MB adsorption by activated carbons from lignocellulosic materials demonstrates a strong correlation with the Langmuir isotherm equation [61].
Furthermore, experimental data were adjusted to the PFO and PSO order kinetic models; however, both biochars were better fitted to the PSO (R 2 > 0.998) (Figure 9). The PSO model describes the adsorption process in a surface monolayer. The results agreed with those observed in adsorption isotherms.

Conclusions
The results showed that the yield of BCM was 28.483%, higher than that of BC (i.e., 26.160%). Both biochars show a highly porous surface, a large surface area, and formly distributed pores and they are mainly composed of C and O, in addition to F and K. BCMFe biochar contains a higher amount of Fe compared to BCM due to its imp nation with Fe ions, successfully achieving the addition of Fe3O4 particles, and XRD a ysis supports this result.
While the ash content of BCM was 4% lower than BCMFe (i.e., 8%), there were more inorganic compounds in BCMFe. Both biochars presented carbonyl and alip groups on the surface. The TGA revealed that BCM carbonizes at 500 °C with a we loss of 93.8%, while BCMFe carbonizes at 650 °C with a weight loss of 78.6%. The a indicates that BCMFe showed higher thermal stability.
It was possible to obtain a material capable of removing the organic pollutant m ylene blue, with a maximum adsorption capacity of 23.17 mg/g and 39.66 mg/g for B and BCMFe biochar, respectively. The modified biochar (BCMFe) showed a higher ads tion capacity (i.e., 71.17%) than BCM.

Conclusions
The results showed that the yield of BCM was 28.483%, higher than that of BCM Fe (i.e., 26.160%). Both biochars show a highly porous surface, a large surface area, and uniformly distributed pores and they are mainly composed of C and O, in addition to Fe, Si, and K. BCM Fe biochar contains a higher amount of Fe compared to BCM due to its impregnation with Fe ions, successfully achieving the addition of Fe 3 O 4 particles, and XRD analysis supports this result.
While the ash content of BCM was 4% lower than BCM Fe (i.e., 8%), there were 4% more inorganic compounds in BCM Fe . Both biochars presented carbonyl and aliphatic groups on the surface. The TGA revealed that BCM carbonizes at 500 • C with a weight loss of 93.8%, while BCMFe carbonizes at 650 • C with a weight loss of 78.6%. The above indicates that BCM Fe showed higher thermal stability.
It was possible to obtain a material capable of removing the organic pollutant methylene blue, with a maximum adsorption capacity of 23.17 mg/g and 39.66 mg/g for BCM and BCM Fe biochar, respectively. The modified biochar (BCM Fe ) showed a higher adsorption capacity (i.e., 71.17%) than BCM.